We propose to to purify to homogeneity the four DNA polymerases known to occur in human milk. The methodology will include the use of ion-exchange and affinity column chromatography as well as velocity sedimentation. Biochemical characterization studies will include: (1) Template primer specificities, (2) Determination of molecular weight, (3) Divalent cation requirements, (4) Determination of sensitivity to N-ethylmaleimide, (5) Determination of the ability of the isolated DNA polymerases to transcribe naturally occurring DNA. We propose to prepare antibodies to the purified milk DNA polymerases. Antibodies to the viral type DNA polymerase (already identified in human milk) will be used to study the immunological relationship of milk reverse transcriptase to the reverse transcriptases of a variety of primate type C viruses. We propose to develop a radioimmune assay for milk reverse transcriptase. This RIA will be used to examine individual human milk specimens, breast duct, fluids and selected benign and malignant breast tissue specimens for the presence of reverse transcriptase. Sera of breast cancer patients will be examined for antibody specific to milk reverse transcriptase.